In recent years, the output power of fiber lasers has been increased. There has been developed a fiber laser apparatus having a plurality of fiber laser units that combines laser outputs from the respective fiber laser units to obtain a high power of several kilowatts. Since such a fiber laser apparatus has a high power, the quantity of light of excess losses, such as losses at a fusion splicing portion or losses caused by generation of higher modes due to a bent of a fiber, may be increased. Such excess losses may increase heat being generated. Heat generation due to excess losses may directly result in a failure such as a breakage of the fiber. Therefore, most of such high-power fiber laser apparatuses are provided with means for detecting a failure of the apparatus, such as a breakage of a fiber.
For example, Patent Literature 1 discloses a method of detecting a failure of a fiber laser apparatus. The method includes detecting, with imaging means (photodetector), a laser beam leaking out of a fusion splicing portion in the fiber laser apparatus when a failure occurs, and thus detecting a breakage of an optical component in the fiber laser apparatus. With this method, however, imaging means (photodetector) needs to be provided for each of portions where any failure is likely to occur. Therefore, if a breakage of an optical fiber occurs at any portion other than the predicted breakable portions, then such a failure cannot be detected. When portions where any failure is likely to occur exist over a wide range of the apparatus, a large number of imaging means are required. Thus, the cost of the apparatus is increased.